1. Field of the Invention
The present invention generally relates to a time division multiplexing (TDM) passive optical network (PON) system for preventing signal collision, an apparatus thereof, and a method thereof.
2. Description of Related Art
The quantity of data transmitted over networks increases along with the increase of network users. In the conventional communication technique wherein data is transmitted as electric signals, network congestion may be caused due to the bandwidth limitation of the electric signals. Thus, optical fiber communication is adopted by many network service providers for providing various network services to network users.
Optical fiber communication offers greater bandwidth than wireless or cable communication using electric signals therefore can transmit data of larger quantity and provide better network services. Presently, passive optical network (PON) is usually adopted by optical fiber communication systems. A PON is easy to maintain and consumes less power because it uses passive devices and requires less switching equipment. Nowadays, many countries are dedicated to the development of PONs such as fiber to the home (FFTH), fiber to the curb (FFTC), and fiber to the building (FFTB) by using optical fiber communication techniques in order to allow network users to transmit and receive data in high speed and large quantity. Accordingly, PON is playing as one of today's major communication techniques.
In a PON, each optical network unit (ONU) has different distance to an optical line termination (OLT) in the central office. Accordingly, the transmission of upstream signals (usually optical signals of 1310 nm) can be controlled in a time division multiplexing (TDM) manner in order to prevent signal collision. As a result, the optical transceiver in each ONU has to be a laser source in burst mode in order to meet the requirement of the TDM transmission manner. The entire PON will collapse if any ONU in the PON turns from the burst mode into a continuous wave (CW) mode due to some module problems. It is impossible to transmit all the upstream signals due to the problem of signal collision.
FIG. 1A is a schematic diagram of a conventional TDM PON 100 in normal operation. Referring to FIG. 1A, the TDM PON 100 includes an OLT 101, a plurality of optical fibers 102, an optical coupler device 103, and a plurality of ONUs 1041˜1044. The OLT 101 is coupled to the optical coupler device 103 via the optical fiber 102, and the optical coupler device 103 is coupled to the ONUs 1041˜1044 via the optical fibers 102. Regarding a downstream signal, the optical coupler device 103 splits the downstream signal so that each ONU can receive a downstream signal; and regarding upstream signals, the optical coupler device 103 couples an upstream signal of each ONU so that the upstream signals of the ONUs 1041˜1044 can be transmitted to the OLT 101 successfully.
The ONUs 1041˜1044 are respectively assigned time slots ts_1˜ts_4. If the ONU 1041 has upstream data, the ONU 1041 has to transmit the upstream data to the OLT 101 in time slot ts_1. Similarly, the other ONUs 1042˜1044 respectively transmit their upstream data to the OLT 101 in their own time slots ts_2˜ts_4. Accordingly, the laser sources in the ONUs 1041˜1044 have to operate in burst mode.
FIG. 1B is a schematic diagram of the conventional TDM PON 100 operating in the condition of signal collision. When the laser source in the ONU 1043 turns from burst mode into CW mode due to some external or other factors such as earthquake, damaged optical fibber module, or other environmental problems, the upstream signals of the ONUs 1041, 1042, and 1044 may collide with the upstream signal of the ONU 1043 so that the TDM transmission manner is destroyed and accordingly the entire TDM PON 100 collapses.
In order to prevent foregoing situation, a method and an apparatus for transmitting a control signal through an OLT for controlling the on or off of each ONU are disclosed in U.S. Patent No. 2005/0244160 A1. However, the OLT circuit has to be re-designed in this disclosure.